1. Field of the Invention
This invention relates to in particular, but not exclusively, a sampling module and a method of sampling one or more analogue characteristic of a power transmission system.
2. Description of the Related Art
An intelligent Electronic Device (IED) uses samples of the analogue voltage and current characteristics of a power transmission system to help protect and control the system.
A conventional arrangement for sampling such analogue characteristics is a, so called, “multiplex system”, as shown schematically in FIG. 1.
A typical multiplex system 10 includes a plurality of inputs 12, each for sampling a respective analogue characteristic.
Each input 12 includes an isolating transformer 14 and an anti-aliasing filter 16 electrically connected in series therewith.
An output of each anti-aliasing filter 16 is connected to an analogue multiplexer 18 which is, in turn, connected in series with a hold filter 20.
The hold filter 20 is connected in series with an analogue to digital converter (ADC) 22.
In use, each isolation transformer 14 steps down, i.e. reduces, the magnitude of the sampled analogue characteristic to a level suitable for analogue to digital conversion by the ADC. In addition, each isolating transformer 14 isolates the remaining portion of the multiplex system 10.
Isolation is required because each input 12 is connected via a step down transformer (not shown) to the power transmission system operating at a higher voltage.
The isolating transformer provides protection against voltages induced in the wiring and differences in earth potential that may exist between the location of the step down transformer and the location of the IED. The isolation provided also protects a human operator and the various other components of the system from the high voltages at the inputs 12.
The analogue characteristic is fed from each isolating transformer 14 through a corresponding anti-aliasing filter 16 to remove noise and high frequency harmonics.
The analogue multiplexer 18 then feeds each filtered analogue characteristic sequentially to the hold filter 20 which stabilises the respective analogue characteristic. This allows the ADC 22 to digitise each characteristic before outputting a digital data stream to a processing module 24.
There are a number of disadvantages associated with the aforementioned multiplex system 10.
Each of the isolating transformers 14 needs to be physically large in order to accurately reproduce the analogue characteristic. As a consequence the system 10 is large and heavy.
In addition, each of the isolating transformers 14 has a limited linear operating range. This results in non-linearity errors which are often at their most severe when the greatest accuracy is needed.
Furthermore, each isolating transformer 14 tends to leak flux. Consequently, close packing of the isolating transformers 14 leads to cross-talk between adjacent transformers 14.
Therefore it is a general aim of the invention to provide a sampling module which is more compact and more accurate than conventional multiplex sampling systems.